During the drilling of a bore hole a flushing fluid is circulated around the borehole to remove the debris generated by the drill bit. This debris is entrained in the flushing fluid and attempts are made to remove all debris from the flushing fluid at the surface so that the flushing fluid can be re-used. The industry term for this flushing fluid is drilling fluid or mud.
Great efforts are made to increase the efficiency of the debris removal prior to re-use, as the drilling performance decreases with increasing debris content. In the worst case excessive debris levels make the mud unusable and it has to be discarded. The performance of a debris-laden mud can be improved by either replacing a portion of the mud, or by performing secondary chemical and or mechanical treatments. The requirement for secondary treatments and/or mud replacement significantly increases operating costs and often has environmental implications.
It is common practice initially to try to remove the majority of the debris by flowing the debris laden mud over and through a device containing a vibrating screen, typically referred to as a shale shaker or shaker. The shale shaker allows the mud to filter through a screen which vibrates in such a manner as to convey the debris along and off the top of the screen (or screens) to its disposal point. The cleaned mud is then either pumped down the borehole to flush out further debris or is sent for secondary processing prior to re-use. The smaller the aperture of the holes in the vibrating screen of the shale shaker the better the screen is at removing fine particles that are the most detrimental to drilling fluid performance.
The shale shaker is the primary method of removing the debris from the mud. Generally it is a robust piece of equipment. However there are several disadvantages to this method:
The screens often plug up with debris getting trapped in the screen mesh resulting in the loss of excess mud with the waste debris. This loss of mud increases costs and may be environmentally unacceptable. To avoid this, coarser screens (screens with larger apertures) are often fitted. This allows more debris to remain in the mud, which in turn increases the amount of additional treatments required to return the mud to its desired properties. In the worst case the majority of the screen area may become covered or plugged with debris, resulting in most of the mud being discarded with the debris. This is often referred to as screen blinding.
The screens do not have a long life, especially when the finer mesh screens (screens with smaller apertures) are used. Therefore it is expensive to run fine screens that remove most of the debris. In fact very often the screens with the smallest apertures are not run because screen replacement costs exceed the additional treatment costs associated with treating the mud to make it fit for purpose. In the worst instances, screen life may be as low as a few hours. Screen wear is compounded by the fact that the debris continually impounds on the length of the screen. Screen wear is highest at the point where the fluid first hits the screen. Screens may therefore be discarded when a large portion of the screen has relatively little wear.
The throughput of the screens, often referred to as the conductance, usually decreases when finer mesh screens are used, as the percentage of wire to open hole area increases. Thus for a given screen area, when higher throughputs are required, it is often necessary to fit larger screens which allow more debris to remain in the mud.
The throughput or conductance through the screens decreases as the amount of debris in the mud increases, as may be the case when drilling rates increase. This is due to the covering of the screen apertures with debris. To avoid the loss of mud it may be necessary to fit larger screens, which again allows additional amounts of debris to remain in the mud.
Screens are expensive and take time to replace. While screens are being replaced, it may become necessary to reduce the circulation rate of the mud, reducing the performance of the drilling operation. Even the few minutes taken to make the change can add significant costs to an expensive drilling operation.
Screen area is limited as the depth of the pool of fluid on top of the screens is limited by the difficulty in transporting debris uphill to be discarded by means of vibration alone.
FIG. 1 illustrates an apparatus according to the prior art, in which the debris laden mud 1 flows downwardly through a vibrating flat screen 2 to filter out the debris. The screen 2 is vibrated by vibrating the whole body of the apparatus using motors 3 with eccentric weights, for example. The cleaned mud exits the apparatus and then generally is ready for re-use. Debris that does not pass through the screen is caught on the screen and is carried by the direction and amplitude of the screen vibration upwardly towards an exit 4. Corrugated shaped screens are often referred to in the industry as pyramid screens. In prior art screens the mud passes either vertically down through the screen, as is the case with the flat screens, or has a downward component to the flow, as is the case with the pyramid screens. More recently arrangements have been proposed where the angle of the screen is increased and may be vertical in parts.
Examples of this general configuration of filter are disclosed in U.S. Pat. No. 4,459,207, WO-A-02 43 832 and WO-A-03 028 907.
The object of the present invention is to provide apparatus for sieving the mud which reduces the disadvantages of prior art equipment.